A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a typical configuration of a lithographic apparatus, the substrate is held on a substrate support during the exposure process. In order to adjust the position of the substrate during an exposure or between exposures, the substrate support is moved. Typically, therefore, an actuator is provided that adjusts the position of the substrate support relative to a reference frame. In general, the position of the substrate relative to the substrate support is fixed and known. Therefore, the position of the substrate may be determined by monitoring the position of the substrate support. It has previously been known to monitor the position of a substrate support using interferometers and/or grating encoders. Such systems are capable of providing highly accurate position measurements. However, the accuracy of such systems may be reduced if the temperature and/or pressure and/or composition of the gas through which the beam of radiation used for the interferometer or grating encoder propagates, varies. For example, in order to provide sufficient accuracy for a lithographic apparatus, it may be desirable to limit the temperature variation to a range of +/−100 to 300 mK. This may be difficult to provide in a lithographic apparatus because there are significant sources of heating, such as the actuator for moving the substrate support and the radiation being imaged onto the substrate. Accordingly, the gas adjacent to the areas of heating are heated. The heated gas may then be pumped around the environment of the lithographic apparatus by the movement of the substrate support.